Generator sizing doubts
Generator sizing doubts
(OP)
Hello,
I'm on a project where we will be using a 410 kw prime diesel generator to run three motors: a 30 Hp, 30 Hp, and 335 Hp. All motors start across the line, and the 335 will start first. The generator can produce 617 peak amps. The big motor has an FLA of 400. I have doubts that the generator will be able to start the big motor. The generator dealer disagrees. Does anyone have some information to convince me that the generator can produce enough current to start the big motor?
thanks
I'm on a project where we will be using a 410 kw prime diesel generator to run three motors: a 30 Hp, 30 Hp, and 335 Hp. All motors start across the line, and the 335 will start first. The generator can produce 617 peak amps. The big motor has an FLA of 400. I have doubts that the generator will be able to start the big motor. The generator dealer disagrees. Does anyone have some information to convince me that the generator can produce enough current to start the big motor?
thanks





RE: Generator sizing doubts
I'm sure waross will weigh in with more details.
RE: Generator sizing doubts
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator sizing doubts
I would be concerned as well. I'd suggest a motor starting analysis.
David Castor
www.cvoes.com
RE: Generator sizing doubts
I would like to see some documentation which says the generator can provide 400% peak current for 4 seconds. That would make me feel better about this.
RE: Generator sizing doubts
RE: Generator sizing doubts
Keith Cress
kcress - http://www.flaminsystems.com
RE: Generator sizing doubts
The customer owned an unused generator and was not about to buy another. I explained the amount of load curtailing that would have to be done and he said;
"Fine. We can live with that. Go ahead."
It turned out that although we did have to curtail loading, we could handle quite a bit more load than the software allowed.
But you are pushing the envelope as far as motor starting on a gen set.
I find that equating HP to KVA is acceptable for motors on generators. The combined efficiency and PF is often close to 80% and using .746 rather than .8 is a close approximation.
On that basis I would want to see 670 KVA or 536 KW on the gen set.
But all is not lost. Almost all diesel generators in that size range use an Automatic Voltage Regulator (AVR) which incorporates a feature known as Under Frequency Roll Off (UFRO).
When the 335 HP motor starting load hits the generator, the speed and frequency will drop quite a bit. UFRO generally allows about 3 Hz frequency drop and then starts dropping the voltage in proportion to the frequency drop. This gives a constant Volts per Hertz ratio. (With a small 3 Hz. offset).
What we have here is the poor mans VFD. The reduced voltage avoids the high currents expected with DOL starting.
Possible issues.
Trips> Starting current will still be high but not as high as DOL on the grid. You may have to adjust the current trip settings.
Low voltage> You may have to use a UPS to provide control power during heavy starting.
Gen Set duty> If you have a prime rated set, add 10% to the generators KVA and KW ratings and use the higher ratings to predict allowable motor starting loads. For a standby rated set you must use the nameplate ratings.
Frequency> If your set has only basic controls you are probably good to go. If you have a more sophisticated control scheme you may have to disable under frequency tripping.
Bottom line> You project may well be doable, but be guided by itsmoked's comment regarding the big picture. When you push the envelope this much, you have to have all the factors. Being on site is even better.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
However this thing starts, that generator is going to produce a big groan on start up.
RE: Generator sizing doubts
Schneider recommends a current limit of 400% FLA for their solid state soft starts in crusher applications, so try using this number in your software. Make sure if you use a solid state starter that you include an automatic bypass to reduce harmonics on the generator.
RE: Generator sizing doubts
RE: Generator sizing doubts
Since the motor always starts unloaded, I suspect it will start OK.
You do need to give some thought to the motor starter, if there is one. The contactor can start to dropout or chatter around 85% voltage on the control power. I suspect your voltage at the starter will be less than that initially.
David Castor
www.cvoes.com
RE: Generator sizing doubts
Any specific reasons why a soft start would cause problems? I was under the impression that as long as it is a permanent magnet generator there should be no problems with the voltage regulator circuits.
RE: Generator sizing doubts
That's what I was trying to say. David used a lot less words.
You may consider going one step further and closing the starter for the large motor before the generator is started. You will need a source for control power to do this. A UPS for just the crusher controls should work well.
By all means, don't try to start the large motor with a 30 HP motor already running.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
I like the idea of connecting the motor prior to starting the generator. If the motor is unloaded and the field is applied gradually, so there is a longish voltage ramp, you could get very little "inrush".
RE: Generator sizing doubts
Seems that there is a (not often but significant!) number of times that rocks will be in the crusher (after an emergency shutdown, jam, generator trip, crusher fault) and can't be hand-removed before re-starting.
RE: Generator sizing doubts
Not trying to get off topic, but I don't agree with this as a blanket statement. Gensets with three phase averaging AVR's (as opposed to single phase half-wave regulators) are commonly used in applications with high harmonics to prevent issues. Also, with a bypass on the soft starter, harmonic heating of the windings would not be an issue.
I would like to see any reports to the contrary, as I will be commissioning one such system soon and would like to be prepared for potential headaches.
RE: Generator sizing doubts
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
RE: Generator sizing doubts
RE: Generator sizing doubts
Thanks
Bill
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
RE: Generator sizing doubts
Another issue may be a d'Arsonval based meter giving an inaccurate indication on a distorted wave form.
On the other hand, if the frequency is being dragged down I want to see a voltage drop.
But, I won't challenge Lionels experience.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
RE: Generator sizing doubts
They started almost well on the generators.
By that I mean that we had two generators in parallel and needed both generators to start the large motors even with the soft starts.
One generator had a hydraulic governor and the other had an electronic governor.
When the motors started tripping out on phase angle error we would start the generators but if we tried to start immediately the hydraulic governor could not respond fast enough to prevent the electronic governed machine from hogging the load and tripping the main breaker on the gen-set. After warming up for 15 or 20 minutes the oil would be thin enough that gen-sets would share the load well enough to avoid tripping the breaker.
Al;though we had phase angle issues on the grid we never had frequency issues when running on the gen-sets.
A VFD with a passive front end is just rectifying the AC. It should be even more forgiving of frequency excursions than a soft start.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
RE: Generator sizing doubts
Generators in parallel.
1> The load (KW) that each generator will accept depends on the throttle setting of the engine or prime mover. This may cause a slight interaction with the frequency.
The Classic control scheme is to run the governors in droop mode. At no-load the generators run 3% fast and drop to nominal speed/frequency at full load.
2> The VAR sharing depends on the voltage setting. There is a scheme called cross current compensation or the quadrature circuit which biases the AVRs to compensate for small inequalities in AVR settings. In practice, with a normal load, the set in your example that has the higher voltage setting will take a larger share of the VAR demands. If your example voltages are stand-alone voltages, the cross currrent compensation will work to reduce the difference when the sets are in parallel.
3> Do the imbalances cause damage? Usually small imbalances cause one set to take more than its share of the load. Governor issues cause KW sharing issues. Voltage setting issues cause VAR sharing issues.
4> One set may take more than its share of the KW load while the other set takes more than its share of the VAR demand. There is very little interaction if any between speed settings and voltage settings on paralleled sets.
BUT
If the sets are heavily loaded, sharing issues may cause one set to be overloaded. This may be a current overload (with heating issues), a mechanical overload with prime mover issues or both.
Governor issues may cause mechanical overloading which may lead to overcurrent issues.
Voltage setting issues generally lead to overcurrent and heating issues.
And, a severe unbalance under light loading may cause the unloaded set to "Motor". This does not generally cause immediate damage but if the condition persists may lead to wet stacking or slobbering. This is a carryover of lube oil into the exhaust system. Some piston rings particularly when old, do not seal well without the force of combustion. Also, any lube that passes the rings will eventually be pushed out into the exhaust system instead of being burned with the fuel in the combustion chamber. When the engine is again put under load, the wasted lube oil may be ignited in the exhaust system, it may be expelled, (messy) burning lube oil may be expelled from the exhaust or an explosion inside the exhaust system may result.
This used to happen when the operator tried to stop a generator but forgot to open the circuit breaker first. A common design feature is a reverse power relay/trip to avoid this event. The last time I saw this happen I replaced a faulty reverse power relay.
Another possible issue may be damage to the fuel injection system. Some injection pumps depend on the flow of fuel for lubrication and extended operation with no fuel flow may cause damage. While most diesel engines will run on jet fuel, it will also lead to damage to some injector pumps due to insufficient lubricity.
Bill
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"Why not the best?"
Jimmy Carter
RE: Generator sizing doubts
RE: Generator sizing doubts
RE: Generator sizing doubts
These hold the voltage up very well as long as the frequency holds up. When you are starting large motors on a gen set you often have a frequency drop. Depending on the size ratio between the generator and motor this may be until the governor opens up enough to pick up the load or it may be sustained longer.
If the generator is lightly loaded and turbo charged, you may have a frequency drop while the turbo spools up.
When the frequency drops more than 3 Hz the UFRO function of the AVR will drop the voltage regardless of the voltage available from the PMG exciter supply.
Note: The Permanent Magnet Generator supplies power to the AVR to avoid voltage collapse on faults. The main generator is not changed and may be excited with a conventional AVR in an emergency while waiting for parts should the PMG add-on fail. There may be protection and block loading issues but the set will generate and supply most loads.
Bill
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"Why not the best?"
Jimmy Carter